Papers by Keyword: Torsion Testing

Abstract: The paper is devoted to the practical implementation of the new torsion testing method for studying rheological properties of materials in a hot state. This method involves the testing of cylindrical samples in the grips of a test setup, the angular velocity of which changes exponentially. The testing mode allows you to restore the hardening curves of a material according to the test results. This article aims to formulate the requirements for possible ways to implement the proposed testing method, and presents two different ways to obtain the specified exponential testing mode. The experience of their use on the test setup in the Ural Federal University indicates the feasibility of the new testing method, as well as the possibility of a smooth transition to the specified testing mode.

Abstract: The present work is focused in the examination of the torsional behaviour of composite tubes by a combined experimental and numerical approach. Glass and carbon composite tubes were manufactured by the filament winding technique. All the tubes were fabricated with glass and carbon Fiber orientation at ±45°. The effect of the torsional loading on the mechanical strength of the glass and carbon composite tubes was initially studied experimentally. Angular velocity of 5° per min was used as torsion test speed while torque-twisting angle changes were recorded. The torsional behaviour of composite tubes was also simulated using Finite Element Analysis (FEA). An elastic orthotropic composite model was used for the simulations. The normal and shear stress contours were obtained from the FE models, while the theoretical relation of the torque versus the twisting angle was calculated. Comparison of the numerical and experimentally obtained results has shown a relatively similar torsional behaviour.

Abstract: Seven-pass strip rolling simulations were carried out on a 0.06%C and a 0.09%C-0.036%Nb steel. The rolling loads (mean flow stresses or MFS’s) did not increase as the temperature decreased during the simulation. This is ascribed to the occurrence of dynamic transformation. The simulation results are compared to the high temperature flow curves determined on eight plain C and Nb-modified steels in both compression and torsion and at a series of temperatures and strain rates. When the associated MFS’s are plotted against inverse absolute temperature in the form of Boratto diagrams, the stress drop temperatures, normally defined as the upper critical temperature applicable to rolling, A_r3*, are shown to be about 40 degrees above the paraequilibrium and about 20-30 degrees above the orthoequilibrium A_e3’s. These drops are ascribed to the dynamic transformation of austenite to ferrite, a softer phase. The characteristics of the ferrite produced dynamically are described and the transformation is shown to be displacive in nature, leading to the appearance of fine Widmanstätten plates. These plates coalesce into polygonal grains on further deformation and on holding.

Abstract: Alloying high-chromium steels with Nitrogen leads to increase in strength, fatigue life and corrosion resistance, but reduce ductility and could induce cracks formation during forging. In order to address these problems, the hot working response of a high Nitrogen martensitic stainless steel (Fe-16.2%Cr-1.1%Mo-0.33%N-0.34%C) has been investigated by means of hot torsion tests up to rupture, in the temperature and strain rate ranges of 900-1200°C and 0.005-5 s-1 respectively. The peak stresses of the flow curves were related to strain rate (e&) and temperature (T) by the well known sinh equation. The ductility and the safe working conditions were presented in terms of processing map. The microstructure of the steel in the quenched state after deformation was analyzed by means of optical microscopy; the differences in term of morphology and distribution of the various constituents were discussed.

Abstract: The influence of severe warm deformation on the microstructural evolution and flow behavior of a plain carbon steel has been investigated through torsion testing. The specimens were severe deformed within the ferritic region up to strain of 70 with constant strain rate of 0.1 s 1 − . Microstructural evolutions have been investigated using high resolution electron backscatter diffraction (EBSD). True stress-true strain curves exhibit a single and smooth maximum, followed by a slow but significant softening stage. A steady state is observed at very large strains. This finding suggests that the relative balance between comparable work hardening and dynamic work softening results in the occurrence of warm ductility during large deformation. The initial grain structure was equiaxed, but at low strains the grains become elongated in the torsion direction. However, at large strains the grain aspect ratio decreases and finally, further straining leads to the formation of new fine grains with high-angle boundaries, which become more equiaxed than the previous fragmented structure. The flow stress, as well as all the average microstructural parameters, then remains independent of strain. The mechanisms operating during such warm flow behavior and structure changes are discussed in detail.

Abstract: Warm deformation is one of the promising hot rolling strategies for producing thin hot rolled steel strips. A better understanding of the microstructure evolution during warm deformation is important for a successful introduction of such processing into the industrial production. In the present research, the effect of deformation strain on the ferrite microstructure development in a low carbon Ti-microalloyed steel was investigated through warm torsion testing. Microstructural analysis with optical microscope and electron back-scattering diffraction was carried out on the warm deformed ferrite microstructures. The results show that at the early stage of deformation an unstable subboundaries network forms and low angle boundaries are introduced in the original grains. Then, with further straining, low angle boundaries transform into high angle boundaries and stable fine equiaxed ferrite grains form. It was considered that dynamic softening and dynamically formation of new fine ferrite grains, with high angle boundaries, were caused by continuous dynamic recrystallization of ferrite.

Abstract: Several studies concerning ferrite grain refinement have been developed in recent the last years due to the recognised influence of such microstructures on steels properties. This work was focused on the evaluation of the microstructure and mechanical properties of an ultrafine grained CMn steel obtained by hot torsion deformation and intercritical annealing. After 5 min soaking at 900 and 1200°C, the samples of low carbon steel were quenched and then reheated. Hot torsion deformation was conducted at temperatures of 700 or 740°C. The torsion schedule consisted of 7 isothermal passes leading to a total true strain of ≈1 and generating an ultrafine and inhomogeneous microstructure with grain sizes of the order of 1-m, formed by strain-induced dynamic transformation (SIDT). The samples were heated up to 800oC and held for 1, 2 and 3 h. A more homogeneous microstructure and ferrite grain size were obtained after annealing The microhardness tests showed the reduction in hardness with the increase in annealing time. They also highlighted the effects of the ferrite grain size and the volume fractions of the microstructure constituents.

Abstract: Steels with high amounts of silicon are used in electrical applications due to their low mangectoestriction, high electrical resistivity and reduced energy losses, but they exhibit poor formability. A fundamental study of the workability of such materials using torsion testing may help to understand and to optimise its production. Single deformation torsion tests were carried out on a steel containing 2 wt.-% Si in a temperature range of 800 to 1100°C and strain rates in the range of 0.01 to 2 s-1. A value of 299 kJ/mol was found for the apparent activation energy for hot working after applying the hyperbolic-sine equation to the mean flow stress (MFS) values computed from the test. Multiple deformation torsion tests under continuous cooling conditions were carried out in the same temperature range at strain rates from 0.2 to 1 s-1, the strain per pass and interpass time (determining the cooling rate) were varied. Different critical temperatures, which are of importance for processing this alloy, can be calculated from the dependence of MFS with the inverse absolute temperature; such a method was used to determine the temperature at which recrystallisation stops (Tnr). It was found that this temperature depends on strain rate, pass strain and interpass time. Results of the microstructure analysis of quenched samples are in good agreement with the values of Tnr.

Abstract: In the last years, several studies concerning ultra refinement of ferrite grains have been conducted using different experimental techniques (ECAP, ARB, HPT). The aim of all investigations was to provide an optmized relationship between mechanical properties and microstructure of steels. The present work, likewise, deals with strain induced dynamic transformation of ferrite. Samples of low C-Mn steel were intensely deformed in hot torsion aiming at the production of ultrafine grains of ferrite thereby enhancing the mechanical properties when compared to hot rolled products. After soaking during 5min at 900°C, the samples were quenched and then reheated and submitted to hot torsion deformation at temperatures of 700 and 740°C. The torsion schedule consisted of 7 isothermal passes leading to a total strain of ≈1, generating an ultrafine microstructure with grain sizes of the order of 1µm. The shape of stress-strain curves so obtained suggested that ferrite refinement occurred by dynamic recrystallization. The various constituents present in the microstructure as well as ferrite grain size and morphology were examined by optical and scanning electron microscopy. Microhardness tests were performed to evaluate mechanical properties.